With development of air conditioning technology and enhancement of people's environmental protection awareness, a heat recovery VRF (Variable Refrigerant Flow) air conditioning system becomes increasingly popular in the market. A two-tube type heat recovery system in the related art is one of heat recovery VRF air conditioning systems popular in the present market, which consists of three main parts, namely an outdoor unit, a refrigerant flow direction switching device MS and indoor units. According to the difference between cooling starting capacity and heating starting capacity of indoor units, the system has four operation modes as follows: a pure cooling mode where all of the indoor units conduct cooling; a pure heating mode where all of the indoor units conduct heating; a main cooling mode where the indoor units conduct cooling and heating simultaneously, and the cooling starting capacity is greater than the heating starting capacity; a main heating mode where the indoor units conduct cooling and heating simultaneously, and the heating starting capacity is greater than the cooling starting capacity. When the system operates in the main cooling mode, a refrigerant from an outdoor heat exchanger is a gas-liquid two-phase refrigerant, a gaseous refrigerant is used in heating indoor units to conduct heating, and a liquid refrigerant is used in cooling indoor units to conduct cooling. However, an unacceptable pressure drop is produced if the gas-liquid two-phase refrigerant flows through flow-distribution capillary tubes, so that, the actual system adopts, instead of capillary tubes, flute-shaped tubes for flow distribution, but the flute-shaped tubes has a worse flow distributing effect than the capillary tubes, and will result in a worse performance of the heat exchanger.